A previously proposed fluid pump has a rotatable shaft, an inner rotor, an outer rotor, and a pump housing. The inner rotor has a main body, to which the rotatable shaft is coupled, and external teeth, which are formed in an outer peripheral portion of the main body. The outer rotor has internal teeth for meshing with the external teeth. When the inner rotor is rotated by rotating the rotatable shaft, a rotational force of the inner rotor is transmitted from the external teeth to the internal teeth. Thereby, the outer rotor is also rotated. When the inner rotor and the outer rotor are rotated, the volume of the respective pump chambers, which are formed between the external teeth and the internal teeth, changes. In response to increasing of the volume of the pump chamber, the fluid is drawn into the pump chamber. Thereafter, in response to decreasing of the volume of the pump chamber, the fluid is compressed in the pump chamber and is discharged from the pump chamber (see, for example, JP2013-60901A).
In general, when the temperature of the fluid is decreased, viscosity of the fluid is increased. Particularly, in a case where the fluid is light oil (diesel fuel), a wax component (paraffin) of the light oil is solidified to cause very high viscosity of the light oil at the low temperature (e.g., low winter temperatures). In the case where the viscosity of the fluid is increased, a repulsive force, which is applied from the fluid to the inner rotor, is increased. Thereby, a force (tilting force), which is applied from the fluid to the inner rotor in a direction for tilting the inner rotor, is increased. Thereby, a slide resistance between a radial bearing, which rotatably and slidably supports the rotatable shaft, and the rotatable shaft is increased to cause an increase in the energy loss or generation of damage at a sliding portion between the radial bearing and the rotatable shaft.
With respect to the above point, the inventors of the present application have studied a structure for coupling the inner rotor to the rotatable shaft through a joint member rather than directly coupling the inner rotor to the rotatable shaft. With this structure, the above-described tilting force can be absorbed through resilient deformation of the joint member, and thereby the slide resistance between the radial bearing and the rotatable shaft can be reduced.
However, the inventors of the present application have noticed that the above-described coupling structure poses the following new disadvantage. The pump housing has a rotor receiving chamber, which receives the inner and outer rotors. In the case of the above coupling structure, a joint chamber, which receives the joint member, is required separately from the rotor receiving chamber. A joint receiving chamber side surface of the main body of the inner rotor receives a pressure in the axial direction from the fluid in the joint receiving chamber. Thereby, a surface of the inner rotor, which is perpendicular to the axial direction and is located on an axial side opposite from the joint receiving chamber, is urged against an inner wall surface of the pump housing to cause an increase in the slide resistance of the inner rotor.
That is, in the case where the above-described coupling structure is used, although the tilting force can be absorbed by the joint member, the joint receiving chamber is required. Therefore, the increase in the slide resistance of the inner rotor becomes a new disadvantage.